Separate ways

Published: 02 July, 2014

Wärtsilä recently announced that, as part of a contract agreement with Aker Midsund, it has begun providing design and installation services – together with its proven Vessel Internal Electrostatic Coalescer (VIEC) separation technology – for three oilfields in the North Sea. ODEE spoke with Erik Bjørklund, the company’s acting director for separation technology, about VIEC technology and the cost, time and efficiency benefits it can provide.

Wärtsilä, provider of products, solutions and services to the marine and oil & gas industry, recently signed three contracts with Aker Midsund of Norway to deliver its Vessel Internal Electrostatic Coalescer (VIEC) systems for three North Sea oilfields. The system is claimed to enhance the speed and efficiency of the separation process of oil and water onboard in an environmentally safe manner. The delivery project started in September 2013 and installation began last month (May 2014). VIEC will be used at the Gina Krog, Mariner, and Ivar Aasen oil fields in the North Sea.

The Vessel Internal Electrostatic Coalescer is suitable for both offshore and onshore installations and can substantially improve oil quality while maintaining the produced water quality. It solves the problems often associated with emulsion and capacity limits in three-phase separators, and enhances the speed and efficiency of the separation process by forcing small water droplets to merge and form larger, faster sedimenting drops. Until the development of VIEC, electrostatic coalescer technology had been unavailable for use in three-phase separators.

Rationale

The main purpose of any oil and gas production facility is to separate the oil, water and gas produced into their original phases. This is achieved by a stepwise reduction in pressure down to atmospheric pressure, flashing off the gas and then dehydrating the crude oil to meet its export specification of less than 0.5% BS&W (water in oil).

Conventional gravity-based production separators allow the water to settle at the bottom of the vessel while the oil flows on top of the water phase. A dedicated electrostatic coalescer vessel is normally installed downstream of the production separators to remove the last water fractions. However, according to Erik Bjørklund, Wärtsilä Oil & Gas Systems’ acting director for separation technology, gravity-based separators only remove the free water and have a limited effect on water emulsified into the oil phase. Emulsion layers in the separators can be difficult to monitor and hence difficult to control, which can cause carryover of excess water into the oil outlet. Also, traditional electrostatic coalescer vessels are equipped with uninsulated high voltage electrodes which normally cannot be exposed to gas and more than 10 to 15% water remaining in the oil.

Major development

Bjørklund explained that a major development was introduced in the form of an electrostatic device with insulated electrodes, making it possible to handle 100% water as well as any gas present. This allows such devices to be installed into upstream production separators.

Wärtsilä has delivered and installed Vessel Internal Electrostatic Coalescers (VIEC) into more than 38 test and production separators treating crude oils ranging from API 12–50, with an equal split between retrofits and new builds and covering most of the major national and international oil companies.

Bjørklund explained that the VIEC system introduces an electric field to the water-oil emulsion. “This makes the water droplets attractive to each other, which results in them merging and becoming bigger,” he said. “Stokes Law points out that velocity is proportional to the diameter of the droplets squared (2); in other words, the bigger the droplets the faster they fall, making the whole process of separation more is efficient. In a matter of seconds these droplets merge together to become very much bigger droplets and therefore fall out very much faster. That’s how it works and that is also what makes it efficient; it’s a faster process.”

Bjørklund added that, from an environmentally friendly standpoint, in order to achieve the same results by other means you have to heat the fluid so that it becomes lighter and less viscous. “This heating will demand power provided by gas or other fossil fuels, and if you compare that to an increase in temperature – say, by 30° – you are talking about megawatts of heating, whereas the energy consumption in the VIEC system can be measured in kilowatts – 300 or less. Therefore it can be seen to be considerably more environmentally friendly than other methods of oil and water separation.”

Challenges beyond easy oil

The often-hear phrase ‘Beyond easy oil’ refers to the fact that most of the oil that is easy to extract has already been produced. Bjørklund explained that existing fields are maturing and experiencing more water coming out of the reservoirs with less oil, exceeding the design basis for the production equipment in use. He added that reservoir pressures are dropping, resulting in the installation of pumps, which can cause problems with stable emulsions in the separation process. Oilfield development is becoming increasingly challenging because half of the world’s remaining reserves comprise heavy or extra-heavy oil,” said Bjørklund.

Traditionally, heavy oils are separated by using very large separator vessels and allowing extra-long periods for the water to settle, as well as adding large quantities of chemicals and heating the crude oil to temperatures of up to 150°C. Significant additional operational costs are inevitable. Bjørklund points out that improving the efficiency of the separation process by means of VIEC technology can reduce fluid temperatures below 100°C, optimise separator vessel size, or reduce the use of chemicals. This, says Bjørklund, not only has a positive effect on CAPEX (capital expenditure) and OPEX (operational expenditure), but also on levels of CO2 emissions.

Installation can take less than a week

Production separators are the heart of any oil and gas production facility, offshore or onshore, all the way from the tough conditions in the North Sea to the hot deserts of the Middle East. Any shutdown of these separators will almost immediately hit oil companies’ revenue streams. Bjørklund explained that installing VIEC technology has to be planned carefully. The equipment must be a correct fit at the first attempt and the installation process must be performed quickly to reduce downtime.

The whole installation also has to be performed through a manhole just 18-24 inches wide. As well as being designed to pass through the manhole, the component parts have to be bolted together inside the separator unit.

To secure safe and on-time installation, Wärtsilä performs a full-scale test installation prior to equipment delivery. While installing a VIEC system inside a separator takes just a few days, additional time is required for depressurising and cleaning prior to installation, and pressurising and recommissioning after the installation is complete.”

Allocated tasks

Aker Midsund AS is the Separation Package supplier for all three projects towards the engineering procurement construction (EPC) contract holders. Wärtsilä will in each project supply a complete VIEC system with all required elements, all vessel internal cabling and tubing, penetrators and stainless steel frames, supports, and other items including the power distribution. The systems will enable the quality of the oil and water to be notably improved.

“The design work and equipment installation is carried out by Wärtsilä,” explained Bjørklund, “and we have suppliers that manufacture the tubing, frames and supports etc., as well as the frequency converters. We provide the internal cabling, but not the field cables, and are not vendor-specific in terms of where we source the cables from.”

In terms of the company’s relationship with Aker Midsund, Bjørklund explained that over the past two to three years Wärtsilä has worked on a number of other projects with the engineering, design and technology company, and has always found it to be highly professional, while also being a pleasure to work with.

The systems are scheduled to be delivered to the Gina Krog, Mariner, and Ivar Aasen oil fields by August this year.